Added isDestroying/isDestroyed flags meaning and invariants docs.

Could you please have a look if I described the flags correctly? I think it would be good to have this documented.

In the process of integrating Ember and 3rd parties we found the need to track whether the object is in the destruction process and wanted to have a single flag check. From the source I found the `isDestroying` the right thing to check. It stays `true` ever since the `destroy()` method has been called.